Secretory vesicles which have been isolated from lactating mammary gland will be characterized with respect to the biochemical nature of their membrane and content. Similar characterizations will be made with rough endoplasmic reticulum, Goligi apparatus, plasma membrane and milk fat globule membrane fractions from lactating mammary gland. The presence of enzymes involved in postsynthetic modification of secretory proteins and membrane glycoproteins of milk in the above fractions will be determined. Kinetics of turnover if individual membrane proteins and structural lipids of the above endomembrane components will be elucidated. The roles of calcium and cyclic adenosine monophosphate in milk protein secretion will be studied. Information obtained should lead to a better understanding of the mechanisms of modification and secretion of the constituents of milk and the nature and extent of membrane interconversion and flow in epithelial cells of an exocrine gland.